The present invention relates to a sheet post-processing apparatus for folding a stack of sheets and an image forming apparatus equipped with the same.
Conventionally, in an image forming apparatus such as a copier, printer, facsimile machine or machine combined thereof, there is a apparatus provided with a sheet post-processing device for folding a sheet discharged from an image forming apparatus to obtain a finished booklet.
As such a conventional sheet post-processing device, there is a known device provided with a sheet folding mechanism having a plate-shaped pressing member for contacting a sheet along a folding position and a pair of rotating bodies for drawing in the sheet using the plate-shaped pressing member. FIG. 19 shows an example of such a sheet folding mechanism. A stack of sheets to be folded is transported to a folding position by a pair of stack transport rollers (51 and 52), and the stack stays there in a state that the rollers nip an upstream portion of the stack. In this state, the sheet stack is nipped at an upstream side of the pushing plate 55, and hangs downwardly at a downstream side thereof. Next, the pushing plate 55 is pushed toward a surface of the sheet stack, and the pair of the folding rollers (57a and 57b) rotates to draw in the sheet stack, so that the sheet stack is fold at a folding line. After the folding operation is completed, the nip at the upstream portion of the sheet stack is released to allow the sheet stack to be discharged downwardly.
However, when the pushing plate 55 pushes the sheet stack toward the folding rollers (57a and 57b), the pushing plate 55 may contact the sheet stack at a position slightly shifted from the folding line. This is because the pair of the stack transport rollers (51 and 52) nips the upstream portion of the sheet stack and the downstream portion of the sheet stack hangs downwardly. Therefore, the downstream portion of the sheet stack is pulled toward an upstream side as the pushing plate 55 pushes the sheet stack. For example, as shown in FIG. 19, when the pushing plate 55 pushes the sheet stack at a predetermined folding point (point A), edges of the sheets at the downstream side are not nipped, and are pulled toward the upstream side, thereby shifting the actual folding point toward (point B). Also, an amount of the shift depends on the number of the sheets and a type of sheet to be folded. Therefore, there is a problem in which it is difficult to accurately fold the sheet stack at a desired folding point.
In view of the aforementioned problem, the present invention has been made, and an object of the present invention is to provide a sheet post-processing apparatus and image forming apparatus equipped with the same in which a sheet stack can be accurately folded at a predetermined folding position.